1. Field of the Invention
The present invention relates to a secondary battery, a method of making the secondary battery, and, more particularly, to a secondary battery with an improved finishing tape, preventing the formation of air bubbles under the tape when a jelly roll electrode structure of the secondary battery is wound.
2. Description of the Related Art
Lithium secondary batteries are capable of charging and discharging and are rapidly developing because of their high operating voltage and their excellent energy densities per unit weight, compared to nickel-cadmium (Ni—Cd) batteries and nickel-hydride (Ni—MH) batteries. Lithium secondary batteries can be classified into liquid electrolyte batteries and polymer electrolyte batteries according to the kind of electrolyte used. In general, a battery using a liquid electrolyte is referred to as a lithium-ion battery, and a battery using a polymer electrolyte is referred to as a lithium polymer battery.
Specifically, a secondary battery using a liquid electrolyte employs an jelly roll electrode structure having positive and negative electrode sheets respectively coated with positive and negative electrode active materials with a separator interposed therebetween. This structure is wound in a spiral referred to as a jelly roll shape. An electrode assembly having the jelly roll shape is housed in a battery case, such as a can or pouch, and an electrolytic solution is injected into the battery case.
In winding the positive electrode sheet, the separator, and the negative electrode sheet, which are sequentially stacked, it is necessary to complete the wound electrode jelly roll with an adhesive tape so that the jelly roll does not unwind. Usually an adhesive tape is applied during this finishing process. The tape, generally called a finishing tape, has an adhesive side adhered to the outer layer of the battery structure and to itself to hold the jelly roll structure in a wound state.
Referring to FIG. 1A, a jelly roll electrode structure 10 includes a finishing tape 20 to hold an outermost end 16 of the jelly roll structure 10. That structure has a positive electrode tab 12 and a negative electrode tab 14, and a finishing tape 20 is kept wrapped around the jelly roll structure and is the outermost end 16. More generally, as shown in FIG. 1A, the finishing tape 20 is attached before winding the jelly roll and the finishing tape 20 is used to wind the jelly roll 10. In addition, the tape provides electrical insulation between the jelly roll 10 and the inside of the battery case (not shown) in which the jelly roll is placed.
The finishing tape 20 is made from a thin, polymeric film. As a result, air may be trapped at an adhered surface between the finishing tape 20 and the jelly roll 10. The air entrapped portions 22 produce air bubbles, as shown in FIG. 1B. The air bubbles make the surface of the jelly roll non-uniform, increase battery volume, and can interfere with insertion of the jelly roll 10 in a battery case and be ruptured in placing the jelly roll in a case. The entrapped air portions 22 occur because the finishing tape 20, made, for example, from a thin polyvinyl material, has a trailing end adhered to the jelly roll 10 before a leading end is adhered to the jelly roll 10, so a continuous taping process impossible.
Japanese Patent Laid-open Publication No. 2001-155774 discloses a non-aqueous electrolyte secondary battery in which a current collector for a positive or negative electrode is exposed, without winding a separator around the outermost portion of an electrode. A finishing tape is wound, leaving a portion of the electrode assembly exposed. However, the finishing tape of the non-aqueous secondary battery still has the problems described above.
Japanese Patent Laid-open Publication No. 2001-185224 discloses a secondary battery having a narrow finishing tape for winding an electrode assembly widthwise and lengthwise. However, taping lengthwise makes the manufacturing process complex, and entrapped air portions still remain, even if the finishing taping is narrow.
To effectively expel gas generated in an electrode assembly during charging and discharging cycles, Japanese Patent Laid-open Publication No. 2001-273931 discloses a battery having an insulating tape with exhaust holes. The tape is attached to the lower portion of an electrode assembly. However, the insulating tape is not a finishing tape that can result in trapping air in finishing of a jelly roll electrode structure.